Gas turbines have been widely used to provide power for electric utilities usually for standby or peaking power. Because the thermal efficiency of the gas turbine by itself is rather low due to the high exit gas temperature, the gas turbine is often combined with a heat recovery steam generator and a steam turbine to produce additional electricity. As a combination of a gas turbine cycle and a steam turbine cycle, these systems are referred to as "combined cycles".
All fossil-fuel-fired combustion processes have the potential for producing nitrogen oxides (NO.sub.x) including primarily NO with some NO.sub.2. This also applies to the combustion process in a gas turbine where there are high temperatures and high excess air levels. Therefore, the exit gas from the gas turbine which flows into and through the heat recovery steam generator contains a significant quantity of NO.sub.x. The current global emphasis on environmental protection and pollution abatement has fostered the development of techniques for reducing the emissions of the oxides of nitrogen because they participate in complex chemical reactions that lead to the formation of photo-chemical smog in the atmosphere. One such technique is known as selective catalytic reduction which uses a catalyst and a reactant, such as urea or NH.sub.3 gas, to disassociate NO.sub.x to molecular nitrogen gas and water vapor. Since NO.sub.x is in excess of 90 percent NO, the dominate reaction using ammonia is: EQU 4NO+4NH.sub.3 +O.sub.2 .fwdarw.4N.sub.2 +6H.sub.2 O.
In addition to being used with conventional fossil-fuel-fired steam generators, these selective catalytic reactors have also been used with combined cycle systems with the selective catalytic reactor actually comprising a section within the heat recovery steam generator. For the effective operation of the selective catalytic reactor, it is important that the reactant be thoroughly mixed with the flue gas on a localized basis over the entire cross-section of the inlet to the selective catalytic reactor. This requires a large number of injection nozzles with their associated piping and usually a mixing grid at the inlet to the selective catalytic reactor section in order to assure uniform reactant dispersion. Of course, this all adds cost to the system.